fix: handle disconnected Meter Widget beat input

[Piyushrathoree]

Summary

• Prevented the Meter Widget from dereferencing a missing beat-value block.
• Made the Reset action safe when meter inputs are disconnected.
• Added regression coverage for an unplugged beat-value input.

Issue

Fixes #8006

Testing

• npx jest js/widgets/__tests__/meterwidget.test.js --runInBand
• npm run lint
• npx prettier --check js/widgets/meterwidget.js js/widgets/__tests__/meterwidget.test.js
• npm test

Manual testing confirmed that the Meter Widget opens without the original TypeError.

• Bug fix
• Tests

[codecov]

Codecov Report

❌ Patch coverage is 23.07692% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.14%. Comparing base (51c0185) to head (9233071).

Files with missing lines	Patch %	Lines
js/widgets/meterwidget.js	23.07%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8007      +/-   ##
==========================================
- Coverage   62.15%   62.14%   -0.01%     
==========================================
  Files         187      187              
  Lines       58315    58317       +2     
==========================================
  Hits        36244    36244              
- Misses      22071    22073       +2     

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[walterbender]

;Maybe you can include a screenshot of the test case you are addressing here. Not obvious to me what to test.